The effect achieved by any type of display is influenced by the sensitivity of the human eye to different colours. FIG. 1 illustrates generally the photoptic curve, which plots the eye""s sensitivity across the visible spectrum. Broadly, the human eye is more sensitive to green light and less sensitive to red and blue light. This means that for red, green and blue light emitters to be perceived to be of the same intensity the red and blue emitters must emit more brightly than the green emitter. This generally requires that the red and blue emitters must consume more power than the green emitter.
Most multi-colour display devices comprise an array of pixels, each of which comprises red, green and blue light-emissive zones. The brightness of the light-emissive zones of each pixel are controlled independently by a display controller unit to make each pixel emit a combination of intensities of red, green and blue light that is perceived by a viewer as the desired colour.
Many display devices are used in applications where power consumption is a critical factor. Examples are in battery-powered equipment such as mobile phones and portable computers. In these applications there is a particular drive to reduce the power consumption of the display (as measured in lumens/Watt). However, since extra power is needed to make the red and blue emissions appear to be as intense as the green there is also a need to address the problem that is caused by the eye""s relative insensitivity to some colours
Numerous technologies are available for constructing display devices, and all face this problem.
One specific class of display devices is those that use an organic material for light emission. Light-emissive organic materials are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene (xe2x80x9cPPVxe2x80x9d), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinolino)aluminium (xe2x80x9cAlq3xe2x80x9d). In a practical device, one of the electrodes is typically transparent, to allow the photons to escape the device.
FIG. 2 shows the typical cross-sectional structure of an organic light-emissive device (xe2x80x9cOLEDxe2x80x9d). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent first electrode 2 such as indium-tin-oxide (xe2x80x9cITOxe2x80x9d). Such coated substrates are commercially available. This ITO-coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and a final layer forming a second electrode 4, which is typically a metal or alloy. Other layers can be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.
According to the present invention there is provided a display device comprising an array of light-emissive pixels, each pixel comprising red, green and blue light emitters and at least one further light emitter for emitting a colour to which the human eye is more sensitive than the emission colour of at least one of the red and blue emitters.
There may suitably be one or more further light emitters in addition to the one mentioned above, for emitting additional colours to which the human eye is more sensitive than the emission colour of at least one of the red and blue emitters.
Where the emission colour of any of the further light emitters lies between the emission colours of the red and green emitters then the emission colour of that further light emitter is preferably one to which the human eye is more sensitive than to the emission colour of the red emitter. Where the emission colour of any of the further light emitters lies between the emission colours of the blue and green emitters then the emission colour of that further light emitter is preferably one to which the human eye is more sensitive than. to the emission colour of the blue emitter.
Preferably the emission colour of at least one of the further emitters lies between those of the red and green emitters and the emission colour of at least another one of the further emitters lies between those of the green and blue emitters. In one preferred configuration there are five emitters: the red, green and blue emitters and two further emitters, one having an emission colour that lies spectrally between those of the red and the green emitters (for example an orange emitter) and one having an emission colour that lies between those of the green and blue emitters (for example a light blue emitter).
The display device preferably comprises a display controller for controlling emission from the display. The display controller may suitably receive a signal defining a desired visual output. In response to that signal the display controller may suitably control the brightness of each light emitter of each pixel of the display so as to cause the pixel to be perceptible by a viewer as emitting an appropriate colour. By this means, preferably the pixels in combination display the desired visual output.
The display controller preferably controls the light emitters so as to improve the power efficiency of the display device and/or to minimise (or substantially minimise) the power consumed by each pixel in order to display the desired colour. To achieve this it may control the further emitters to emit in preference to the red and/or blue emitters. This may not be appropriate for some emission colours, so where the colour to be displayed by a pixel cannot be produced without use of the red and/or blue emitters then emission from one or both of the red and blue emitters will be used.
Preferably each light emitter is independently controllable by the display controller. This may be by means of an active matrix addressing scheme, in which case there is preferably thin film active matrix circuitry in the display device, or less preferably by means of a passive matrix addressing scheme.
Each light emitter may use an organic or an inorganic material for light emission. Suitably each light emitter comprises light-emissive organic material, such as light-emissive semiconductive polymer material.
Different materials may be used to generate each emission colour.
The blue light emitter is preferably a deep blue light emitter. The red light emitter is preferably a deep red light emitter. Examples of suitable colours for the blue and red emitters are around x=0.15, y=0.05 and x=0.63, y=0.33 in the 1931CIE scheme.
The structure of the display device may be such that each light emitter comprises a region of light-emissive material. On either side of that region there may be a charge carrier injection layer or electrode. At least one of the electrodes is preferably light transitive, The electrodes may be linear. One electrode may make contact with more than one light emitter. The device may be substantially planar. The electrodes on both. faces of the device may be linear. The electrodes on one face of the device may be orthogonal to those on those other face of the device. There may be one or more charge transport layers located between the electrodes and the light-emissive material.
Some preferred materials for components (where present) of the display device are as follows:
One of the electrodes (the hole-injecting electrode) preferably has a work function of greater than 4.3 eV. That layer may comprise a metallic oxide such as indium-tin oxide (xe2x80x9cITOxe2x80x9d) or tin oxide (xe2x80x9cTOxe2x80x9d) or a high work function metal such as Au or Pt. The other electrode (the electron-injecting electrode) preferably has a work function less than 3.5 eV. That layer may suitably be made of a metal with a low work function (Ca, Ba, Yb, Sm, Li etc.) or an alloy or multi-layer structure comprising one or more of such metals together optionally with other metals (e.g. Al). At least one of the electrode layers is suitably light transmissive, and preferably transparent, suitably at the frequency of light emission from one or more of the light-emissive regions.
The or each charge transport layer may suitably comprise one or more polymers such as polystyrene sulphonic acid doped polyethylene dioxythiophene (xe2x80x9cPEDOT-PSSxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene)) (xe2x80x9cBFAxe2x80x9d), polyaniline and PPV.
The or each organic light-emissive material may comprise one or more individual organic materials, suitably polymers, preferably fully or partially conjugated polymers. Suitable materials include one or more of the following in any combination: poly(p-phenylenevinylene) (xe2x80x9cPPVxe2x80x9d), poly(2-methoxy-5(2xe2x80x2-ethyl)hexyloxyphenylenevinylene) (xe2x80x9cMEH-PPVxe2x80x9d), one or more PPV-derivatives (e.g. di-alkoxy or di-alkyl derivatives), polyfluorenes and/or co-polymers incorporating polyfluorene segments, PPVs and related co-polymers, poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene)) (xe2x80x9cTFBxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene)) (xe2x80x9cPFMxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene)) (xe2x80x9cPFMOxe2x80x9d), poly (2,7-(9,9-di-n-octylfluorene) (xe2x80x9cF8xe2x80x9d) or (2,7-(9,9-di-n-octylfluorene)-3,6-Benzothiadiazole) (xe2x80x9cF8BTxe2x80x9d). Alternative materials include small molecule materials such as Alq3.
According to a second aspect of the present invention there is provided an electronic device comprising a display device as described above.